1. Field of the Invention
The present invention relates to a photoelectric conversion processing apparatus. More particularly, the present invention relates to a photoelectric conversion processing apparatus provided with a photodetector such as a silicon photodiode having a sensitivity which exhibits little dependency on temperature so that an output signal thereof is proportional to incident energy.
2. Description of the Prior Art
A silicon photodiode (SPD) is a photodetector which provides an output signal i in good proportion to incident energy I (i.e., .gamma.=1 in the case of i=k.multidot.I.sup..gamma. where k is a proportion constant and .gamma. is an exponent for intensity of incident energy) and has a sensitivity exhibiting insignificant dependency on temperature. However, it is difficult to utilize a silicon photodiode as a photodetector for detecting incident energy having wavelength of more than 1 .mu.m, for example, 2 .mu.m, since its sensitive wavelength range (i.e., spectrum sensitivity) extends from the visible region only up to approximately 1 .mu.m.
Therefore, in order to form a photodetector for detecting incident energy having wavelength of 1 to 3 .mu.m for example, a device of PbS is conventionally used. The PbS device has a sensitive wavelength range from the visible region to approximately 3 .mu.m and it has advantages that incident energy of longer wavelength can be detected compared with a silicon photodiode and that cooling by liquid nitrogen is not needed.
However on the other hand, the PbS device has disadvantages as described below. An output signal i of the PbS device is not exactly proportional to incident energy I (i=k.multidot.i.sup..gamma.(I)). More specifically, the exponent .gamma. is dependent upon the incident energy I. In addition, the PbS device has sensitivity exhibiting significant dependency on temperature. Thus, there is considerable drift in temperature. Furthermore, since it is of a photoconductive type, bias voltage is required. The comparison of the advantages and disadvantages of a silicon photodiode and a PbS device is as shown in the following Table.
______________________________________ Sensitive Proportion Dependency on Wavelength Constant Temperature ______________________________________ SPD short .gamma. = 1 not significant PbS long .gamma. = .gamma.(I) significant ______________________________________
As is evident from the diagram of incident energy and output voltage characteristics of three PbS devices shown in FIG. 1, the exponent .gamma. shows considerable irregularities dependent on the individual PbS devices. In addition, there is further involved a serious fundamental problem that the exponent .gamma. in the same PbS device varies dependent on the intensity of incident energy (.gamma.=.gamma.(I)). Consequently, in order to measure incident energy by conversion from an output signal of a PbS device, it is necessary to obtain, by carrying out experiments, the exponent .gamma. of each PbS device with respect to various values of incident energy intensity.
In addition, as is evident from the diagram of the temperature characteristics for sensitivity of PbS device shown in FIG. 2, the temperature coefficient shows considerable irregularities dependent on the individual PbS devices and there is further involved a fundamental problem that the temperature coefficient varies dependent on temperature even in the case of the same PbS device. Consequently, in order to apply temperature correction to an output signal, it is necessary to obtain, by carrying out experiments, the temperature coefficient of each PbS device with respect to various temperatures on the PbS devices.
Thus, it is extremely troublesome to obtain various exponents .gamma. and temperature coefficients by experiments and it is further difficult to make suitable corrections based on the thus obtained values in a real measuring operation.